Electron microscope



y 1947. c. H. BACHM'AN ETAL 2,424,789

' ELECTRON MICROSCOPE I Original Filed Dec. 1, 1942 6 Sheets-$heet l 7Inventors:

1g Charles H. Bachmam,

Simon Ramo,

Their Attorney July 29, 1947. c. H. BACHMAN EI'AL ELECTRON MICROSCOPEOriginal Filed Dec. 1, 1942 6 SheetsSheet 2 July 29, 1947- c. H. BAcHMANETAL ELECTRON MICROSCOPE Original Filed Dec. 1. 1942 6 Sheets-Sheet 5Inventors: Charles H. Bachman k Simon Pamo, b flw flwllw yTheirA'btorney.

y 1947- c. H. VBACHMAN EII'AL 2, 24,789

ELECTRON MICROSCOPE ThZYA'btorney By 29,1947. 0. H. BACHMAN ETAL 2,4,789

ELECTRON MICROSCOPE Original Fil ed Dec. 1, 1942 s Sheets-Sheet 5Inventors: Charles H. Bachman, Simon Rama,

Their Attorney.

0. H. BACHMAN'ETAL ELECTRON MICROSCOPE Jul 29, 1947.

6 Sheets-Sheet 6 Original Filed Dec. 1, "1942 lnventors m W m m o H so wm M 8 YM #T C Patented July 29, 1947 2,424,789 ELECTRON MICROSCOPECharles H. Bachman, Scotia, and Simon Ramo, Schenectady,- N. Y.,assignors to General Electric Company, a corporation of New YorkOriginal application December 1, 1942, Serial No. 467,530. Divided andthis application January 26, 1944, Serial No. 519,772

6 Claims. (Cl; 250-.-49.5)

The present application, which is a divisionof I our copendingapplication, Serial No. 467,530, filed December 1, 1942. relates toelectron microscopes, and more particularly to new and improvedstructure for electron microscopes of the electrostatic type. 1 T

In electron microscopes of the prior art, and particularly those whichhave been constructed in accordance with the prior art arrangements andteachings, the structures thereof, particularly the evacuated chamberwithin which the elements are placed, have not been readily adaptable orsusceptible to widespread application due to the fact that the evacuatedchambers have been of relatively large size because of'the type of theelectron lens systems employed, thereby involving relatively large timeintervals between examinations of difierent specimens.

The prior art arrangements have also been's'ubject to the disadvantageof requiring the services":

of skilled operators to efieot frequent re-alinement of the electrodesor focusing apparatus. Where, in the prior art, the magnetic type ofelectron microscope has been employed, in order to obtain accuracy ofthe-image produced and to retain a desired degree of precision in thefocusing effect, it has been necessary to use a large number ofregulating or controlling elements for controlling the voltage orcurrent sup plied to the electrical elements of the microscope. Due tothe above factors the size of the prior art microscopes has beeninordinately large,'thereby restricting the degree of portability andthereby limiting the field of application. i In accordance with theteachings of our-invention described hereinafter, we provide a new andimproved electron microscope which obviates the disadvantages of theprior art arrangements and which aifords simplicity of construction andoperation, and adaptability to a wide field of 'ap- Y plication. Theelectron microscope which we provide is adapted and arranged to permit.-fre quent opening of the evacuated chamber for the-insertion andextraction of specimens to -be investigated and which entails onlythe-lapse of arelatively short interval of time between examinations dueto the fact that the chamber is relatively small. In addition, thespecimen insertion chamber and sealing structure thereof are "-arrangedto permit ease of insertion and extraction of the specimen. Furthermore,theessential elements of the electron microscope, such asthe cathodeconstruction and the electron lenssystem, are readily demountable .forinspection and replacement. Lastly, a unitary electronilensas-- semblyis provided so that the alinement of. the electrodes thereof may beinitially made at the factory, thereby obviating the necessity forfrequent and time consuming re-alinement during use after manufacture.

It is an object of our invention to provide a new and improved electronmicroscope.

' It is another object of our invention to provide new and improvedstructure for an electron microscope wherein the essential elements arereadily demountable and available for inspection and replacement.

' For a better understanding of our: invention, referencem-ay be had tothe following description taken in connection with the accompanyingdrawings, and its scope will be pointed out in the appended :claims.Fig. 1 diagrammatically illustrates an embodiment of our inventionwherein the evacuated chamber containing the lens system is horizontallydisposed and wherein the actuating means for moving the specimen arelocated within the vicinity of the viewing aperture. Fig. 2 is acrosssectional view of the cabinet supporting the 'microscope and shows thedisposition of associated or auxiliary apparatus. Fig. 3 is a crosssectional view of the electron microscope showing the evacuated chamber,electron lens system and cathode supporting structure; Fig. 4

. a perspective view of .a manipulator which may be employed in ourelectron microscope, and Fig. 9 represents the cartridge-type specimencarrier. Fig. 10 represents the structure for holding and positioning anelectron beam restricting'means such as a positionable apertured disk.Fig. 11

. is a view of the specimen insertionmember; Fig. 12 is a detailed viewof the locket which is supported at one end of the holder shown in Fig.11; Fig. 13i-s a view of the sectionalizedhorizontal conductor whichconnects corresponding electrodes of thevarious horizontally disposedelectrostatic lens system. Fig. 14 is an exploded view of themanipulator; Figs. 15 and 16 are front and side views, respectively, ofthe manipulator showing the horizontal motion produceable thereby; Figs.17 and 18 are front and side views, respectively, showing the verticaltransverse motion produceable thereby, and Figs. 19 an-d,20 are Fig. 22is a detailed view of the insertable optical or glass lens which may beused in conjunction with our electron microscope. Fig. 23 illustratesthe manner in which a camera may-be used-to photograph the imageproduced uponthe-fluorescent screen of the electron microscopaandsF-ig.24 is a simplified circuit diagram showing the controllable source ofvoltage for the "electron microscope and the cathode filament supply.

Prior to a detailed description of the embodi ment of our inventionillustrated, it is believed that it may be helpful to present generallycertain fundamental aspects of the electronmicroscope disclosed herein.Generally speaking, the elec- ..tron microscope which'we provide is oneemploying an electrostatic type lens. The microscope comprisesessentially an electron gun-whic'h produces an electronbeam-toillumina-te or irradiate a specimen to be investigated and animaging system which-magnifies the imageproduced by the impingement onthe specimen-into a larger image on a viewing screen such-asafluorescent screen, orupon the'surface. of a photographic plate.

In an electron microscope of the electrostatic type which we provide,the lens focal length-is a function of its physicalsizeandsconfiguration since an increase or decrease in'the electronvelocity is always accompanied by a change-in the electric fieldfocusing action of justrsuch magnitude '01 strength as'to yieldprecisely the same electron paths. 'By the use of *such a system'it-i'sthen notnecessary'to'employ a closely regulated source ofisupplyvoltagefor'the microscope.

Of course, as is 'well known, the desirability of employing .an electronmicroscope in place of a light microscopefor many applications .is' thegreater resolving power incident to .the use :of electron irradiationPas contrasted with: thessmaller resolving power of light microscopes.Although not limited to a particular resolving power 101 magnification,:anelectron microscope built in accordance-with ourinvention operatessatisfactorily having a 200-1Angstrom units resolution and a usefulmagnification of about 10,000 .'As pointed out hereinafter, thismagnification-may be obtained eitherexclusively by thed'esign'of theelectron lens systemitsel'f, or .by'thecombination of a suitableelectron electrostatic lens and an associated light microscope orlenswhich is placed in the vicinity-of .the fluorescent screenat one end ofthe electron microscope.

In the electron microscope described 'hereinafter, certain features. areincorporated by virtue of the configuration and position oftheeler'nents to obtain the desired operation of the electron gun, theirradiation of the specimen, 'andthe desired focusing action of theelectrostatic lens. As concerns the electron gun, this element isconstructed to produce-sufficient current density at the specimenwithout involving anappreciable 70 heating of the specimen bytheelectron beam. On the other hand, the image is illuminated'to asufficient degree of brightness for visual observation with a minimumofcurrent striking the-specimen. It will be apparent that thereisnoadvantage to irradiatea large partof-a'specimen if ,je'o'tive'l'enssystem. This type of limiting is genonly a small portion on the axis isto be imaged. Furthermore, there is no reason to direct electrons at thespecimen if their incident angle is such that in the absence ofdeflection they will consequently be rejected by the objective lens.aperture stop. The contrast obtained at the viewing2 screen orfluorescent screen is the result of distribution of the electronsarriving there. We

'I have found that for very thin specimens the majority of the electronswhich pass through the specimen do so with their slope substantiallyunchanged; consequently, the limiting angle of the electron 'bund-leleaving the objective lens can be -:established principally by theelectron gun ap- ...paratusrather than by the apertures of the ob- Yera'llyeasierto accomplish mechanically by the necessary aperture in theelectron gun, and this necessary aperture may be more favorably locatedthan the objective lens stop so as to obviate the need; of punchingexceedingly small diameter holes in the lens electrodes. Of course, thelens aberrations as well as the particular thickness and material of thespecimen that is under examination, .play an important part indetermining whether the gun angle can serve as an effective ;stop.forthe system, or whether that limit is de- 'termined by the first imagingor objective-lens. It has'been'found that highly successful results areobtained with a small gun angle, and the gun angle produces a morereadily discernible effect uponthe resolving power than does the lensaperture.

The electron microscope described hereinafter deconstructed .to afford"proper relationship between the cathode filament point location, the:filament shield aperture diameter, anode-filament spacing and thespacing and size of 'the other apertures. The spacing and :size of theother @apertures 'of the system depend upon a number of "factors such asthe location of the imaging lenssystem and the spe'cimen,'the .fila-:ment1life, the accelerating voltage, the electron beam-angle -at the"specimen and the extent of magnetic shielding of the field due to thefila- ;ment.current.

concerns the electrostatic electron lenses which magnify the electronimage produced by the impingement of the electron beam on the "specimen,the electrostatic lens system of the electron microscope which weprovide possesses the following "general characteristics. The focallength of the system is positive and relatively short without involvingthe use of excessively small'parts. The in-focus position of the speci-"men is external to the high field region of the lens and the specimenis in a region substantiallyentirely free from electric fields. Theelectrodes o'f the lens system are finely machined or ground-and highlypolished to minimize the insulation problem and field emission at theoper- 'ating voltage. "The lenses are of symmetrical. configurationwith'a single negative potential to ground, namely the cathodepotential, serving to energize thelen's. The axial voltage drop'in thecentral region of the lens is maintained at -an optimum value, therebyminimizing strong magnetic fields and chromatic aberration effects.

Fig. 1-of the drawings represents a perspective view of a cabinet I,which isof a portable nature being mounted on castors 2, and which"houses associated "auxiliary apparatus for an elecftronmicroscope, andon the front of which the electron "microscopetis horizontallypositioned a desk or table part 4 integral with the cabinet I tofacilitate use of the microscope. An end cover 5 encloses the end ofcylindrical cover 3 and is provided with a viewing aperture throughwhich an eye-piece 6 extends. This eye-piece is shown in detail in Fig.22. Therevis also positioned within the vicinity of the eye-piece orviewing aperture a plurality of externally accessible actuating means,such as knurled wheels "I, 8 and 9 which serve to operate a manipulator(described hereinafter) to position a specimen or object to be examined.A specimen insertion chamber terminated in a sealing valve I0 and havingan actuating member I l is conveniently located within easy reach of theoperator at the top of cover 3 and is shown in detail in Fig. 6.

Fig. 2 is a side view showing the disposition of the associatedauxiliary or control apparatus within the cabinet I. A suitable powersupply means 12, including a rectifier and a transformer, is employed inan upper compartment together with voltage controlling means such as arheostatic regulator I3 and a vacuum valve l4. Cooling means for theelectron gun, such as a centrifugal blower I5, is also positioned inthis compartment of the cabinet. In the lower compartment of thecabinet, we employ an evacuation means such as a pair of seriallyconnected pumps i i and Il, both of which are supported by vibrationabsorption means such as springs l8 and I9. Pump It is preferably a highvacuum twostage oil diffusion pump which operates in series with a.mechanical pump, the latter of which furnishes the rough vacuum. Controlmeans for the electrical apparatus and for the pumping apparatus .may bepositioned within the front of the cabinet I. For example, therheostatic regulator l3 of the voltage supply for the electron gun ofthe microscope may be provided with a handle or knob .28 which extendsthrough the wall of the cabinet to make it readily accessible to theoperator. In like manner, referring to Fig. 1, the filament supply forthe cathode of the electron gun may be controlled by a knob or dial 2|also positioned on the front of the cabinet. Likewise. a main powerswitch 22 and a handle 23 connected to'the vacuum valve I 4 throughgearin (not shown) may be made easily accessible to the operator bymounting as indicated.

In Fig. 3 there is illustrated a cross sectional view of the evacuatedchamber, electron gun, the electrostatic lens system, and the specimencarrier manipulator and actuating means therefor. The evacuated chamberwithin which the microscope elements described above are positioned isdefined in part by a metallic cylinder,

such as a brass cylinder 24, the interior surface of which may beadapted to closely engage an insertable unitary electron lens assemblyto be described more in detail hereinafter and which is shown in Fig. 7.As a means for shielding cylinder 24, we provide a laminated metallicshielding structure which includes alternate layers of copper and a highpermeability iron. For example, we may employ cylindrical sheaths ofcopper 25 and 2B and alternate layers of high permeability iron 2'! and28. The sheath which is adjacent cylinder 24 may be either iron orcopper and may be turned over at its ends in order to secure the otherlayers in the position illustrated.

A unitary electrostatic electron lens assembly is defined externally bymeans of a metallic cylinder 29, preferably constructed of brass, theinner surface of which is finely ground and high- 1y polished to closelyengage cylinder 24 and to support therein the elements of theelectrostatic lens system. The inner surface of cylinder 24 and theouter surface of cylinder =29 need not be as precisely machined as theinner surface of cylinder 29. Within the cylinder 29 We provide anelectrostatic lens system which includes a.

-plurality of generally similar lenses 36, 31 and 32. This lens assemblymay be arranged to-have a total magnification of about 1500. Lenses 30and 3| each comprise a pair of longitudinally spaced wafer-likeelectrodes 33 and 3 5 main.- tained at a common or anode potential andwhich. are provided with apertures 35 and 36; respectively. We providean intermediate electrode 32 i also of wafer or disk-like configurationin spaced? relation between outer electrodes 33 and 3E and. which isprovided with an aperture 38 of sii;bstantially larger diameter than theapertures 35 and 36. Electrodes 33 and 3 8 are constructed. to havetheir peripheries finely ground and 1901- ished to closely engage theinner surface of cyl-- inder 29, thereby establishing satisfactorymechanical and electrical connection to the system. Intermediateelectrode 3'! is electrically insulated from cylinder 29 and ismaintained at cathode potential by means of a horizontal conductorconnected thereto and shown in Figs. 7 and 13. Electrode 31 ismaintained in the position illustrated by means of a split insulatingspacer or washer 39 which is supported by and in engagement with theinnner surface of cylinder 28. The insulating washer or spacer 39 may besplit, constituting two segmental parts to facilitate assembly thereofwith the intermediate electrode 31.

It will be noted that the peripheries of apertures 35, 3% and 38 areconvex, thereby reducing to a minimum the tendency to establish unclesired voltag gradients Within the vicinity of the apertures or along theelectron lens system.

The various elements of the unitary electron lens assembly aresusceptible of precise alinement and locking, thereby permitting aninitial alinement of the electrodes at the time Of manufacture andobviating the necessity of re-alinement in the field. For example, theelectrodes of the lenses 3fl32 may be maintained and locked in thedesired illustrated position by means of a plurality of principallongitudinal and annular spacers it-l2, inclusive. In addition, the in--termediate electrode and the outer electrodes may be maintained in thedesired spaced relation by means of smaller annular spacers 43 and 44preferably of metal.

In accordance with our invention, there is provided also a unitarystructure comprising the aforementioned electron lens system and aspecimen carrier 65, shown in Fig. 9, and a specimen manuipulator 46,shown in Fig. 8, Th elements 45 and 46 will be explained hereinafter indetail. The specimen carrier i5 is supported by manipulator 46 which inturn is supported from one end of the lens assembly.

The objective electron lens 32 is essentially of the same electricalcharacteristics as electron lenses 3i) and Si and comprises a pair ofspaced outer electrodes d"! and 48 and an intermediate electrode 49.Outer electrode 48 is provided with an aperture 58 and a radial recess5! adapted to receive a specimen holder shown in Fig. 11, and is alsoprovided with an enlarged aperture adapted to receive one part of thecartridge-type specimen carrier shown in Fig. 9,

As a part of the unitary construction comprising the electron lensassembly and the carrier and manipulator, we also may add an electronbeam defining means, such as an apertured disk 59 and supportingstructure therefor shown in Fig. 10. This supporting structure includesmeans for centering and positioning the disk 50 which may be constructedof molybdenum and which serves to produce a well defined beam for thedesired illumination of the specimen or object which is held by carrier45. The apertured disk 50 is preferably of such dimension to subtend atthe specimen a solid angle in the neighborhood of 10- radians.

A radial specimen insertion chamber 54 is provided in alinement withslot 5| to permit ready insertion and extraction of a specimen incarrier 45. The insertion chamber may be defined by a radial tubularmember 52 preferably constructed of nonmagnetic material and supportingat its upper end a valve 55 for sealing the insertion chamber 56 andalso for sealing the main evacuated chamber of the microscope. Valve 55comprises a removable cover 56 which is engaged by a cross rod whichwhen pressure is exerted thereagainst maintains the cover firmly againstthe upper surface of a cylinder 58, constituting the side wall definingmeans of the valve. If desired, a suitable gasket 59, such as a rubbergasket, may be employed to seal the juncture of the cover 58 andcylinder 58.

Means are provided for supplying to the evacuated chamber of themicroscope air at atmospheric pressure prior to the opening of cover 56.This means may comprise a passage 50 fed by a conduit 6! through whichpre-conditioned or heated air may flow to prevent the condensation ofappreciable moisture on the internal parts of the microscope when it isdesired to extract or insert a specimen. As a means for selectivelycontrolling the flow of the heated air, we provide a valve 62 seated inpassage 69 and actuated by a member 53. A bellows-type sealing member 64is sealed around the adjacent area of cylinder 58 and arm 53. Theactuating mechanism for cover 56 and valve 62 is shown in Fig. 6.

At on end of the cylinder 24 there is located a sealing and. supportingstructure for a filamentary cathode comprising one of the electrodes ofan electron gun, which comprises a filamentary pointed cathode 65 of thehairpin type. There is also provided magnetic shielding structurecomprising apertured wafer-like metallic members or disks B6 and 61which are firmly positioned against the inner surface of cylinder 24.Electrostatic shielding means, such as a transverse metallic planarmember Bl is positioned between members 66 and 61' intermediate theelectron beam apertures of the latter members and the position of alongitudinal conductor to be described presently, thereby defining asubstantially field-free region through which the electron beam passes.Locking means are provided for maintaining all the elements of thesystem in a fixed longitudinal and angular position with respect to theunitary assembly described hereinafter. This locking means may comprisea longitudinal key 68 which engages an abutment of cylinder 29 orextends into a recess provided thereby, and also extends axially throughopenings at the peripheries of disks 66 and 61. Key 68 may also extendthrough annular spacers 69 and '10 which maintain disks 61 and 68 andthe unitary assembly in the desired spaced relation.

Disks (SB-and 61 are provided with apertures H and 12 through which ahorizontal conductor shown in Fig. 13 extends to impress on theintermediate electrodes of electron lenses 39, 3! and 32 a potentialcorresponding to the cathode potential. That is, the cathod structure tobe described hereinafter and the intermediate electrodes are allmaintained at the same potential.

If desired, there may be employed an accelerating anode cup structure 13supported by a transverse disk 13' and extending an appreciable distancelongitudinally around the filamentary cathode 65 so that a suitableaccelerating electric field is provided for the electrons emitted by thecathode.

Exhaust apparatus for the evacuated chamber of the electron microscopemay be connected to a tubular conduit 15 in communication with thechamber and which is also connected to the pumps I6 and il shown in Fig.2.

In order to support the cathode structure and to seal one end of theevacuated chamber defined by cylinder 24, we provide a supporting ring16 which is seated upon a shoulder provided by cylinder 24 and whichalso engages an annular spacer H. A sealing structure is employed andcomprises a compressible washer, such as a rubber washer 18, a metalliccompression ring 19, a threaded retaining ring adapted to engage threadsprovided at the end of cylinder 24, and a. plurality ofoircumferentially displaced means, such as screws 8!, for exertinglongitudinal or axial pressure against the retaining ring 19, therebycompressing washer 19.

A vitreous part 82 is sealed to the supporting ring 18 and serves tosupport and seal the oathode structure. The vitreous part 82 may be ofthe re-e'ntrant type through which a concentric transmission linecomprising an inner conductor 83 and a tubular outer conductor 84extends. The details of the cathode and supporting structime will bediscussed with respect to Fig. 4.

At the other end of the evacuated chamber, particularly at the left-handend of cylinder 24, there is provided an end wall for sealing that endof the chamber and for supporting a viewing screen, such as afluorescent screen 85 coated on the inner surface with a suitablefluorescent material and upon which the magnified image is produced bythe impingement of the electron beam. This end wall comprises anapertured disk 86 whichseats the viewing screen or fluorescent screen85. A suitable seal for the screen -85 may be obtained by means of arubber washer 81 which is compressed by means of a flanged annular ring88, the latter being attached to disk '86. The outer periphery of disk86 is also sealed in a. similar manner by means of a compressible rubberwasher 89, a metallic compression ring 99, a retaining ring 9 I andpressure exerting means such as screw 92 which exert an actual pressureon compression ring 90, thereby compressing the washer 89 and sealingthe juncture.

Means for actuating or controlling the specimen manipulator 45 are alsopositioned at this end of the evacuation chamber within the vicinity ofthe viewing screen. The manipulator 46 may be of the type designed toproduce three degrees of motion mutually at right angles to each other,and may comprise three separate actuating means such as longitudinalextending rods 93, 94 and 95, only two of which are shown 'in' Fig. 3and which are controlled by means of a plurality of screw-thread devices96, 91 and 98, only two of which are shown in Fig. 3. The screw- 7thread devices! is shown in cross-sectional detail and is alsorepresentative of the other'two de-. vices. For example, screw-threaddevice 91 comprises a head 99 which engages the associated longitudinalactuating rod 94 and is controlled in its longitudinal position by meansof atscrew I which is provided with an actuating pin IOI and which hasone end thereof extending through disk 86 to effect an externallyaccessible'connection adapted to receive a flexible cable shown in Fig.21. Each screw-thread device may be supported by a cylindrical memberI02 formed integral with or attached to disk 86 and which is sealed toits associated head 99 by a deformable member such as a bellows I03sometimes defined as a Sylphon bellows.

Reference is now made to Fig. 4 in which the cathode supportingstructure is shown in greater detail. 7 Means are provided forpositioning, that is for centering the point "or hairpin-typefilamentary cathode 65 with respect to the alined' apertures of theelectron optical system. This means is of demountable characterpermitting rapid replacement of the filament and also permits the use ofdiiferent types of filaments in connection with the other elements ofthe electron microscope. A further advantage of the cathode supportingstructure is the'unique arrangement by virtue of which certain portionsof the structure not only serve as a mechanical support for the cathodebut also serve as electrical conducting paths for the conduction ofcathode heating current. j l

A metallic base I04 is sealed at one end to the vitreous part 82 and isprovided with threads to engage threads at one endof the outer tubularconductor 84 to the base I04 which not'only transmits power .to thefilament 65 but also serves to impress through the longitudinalconductor shown in Fig. 13 a negative potential on the filament and theintermediate-electrodes of electron lenses 3032. Base I04 is constructedto afford a flexibleniechnical connection between the inner conductor 83of the concentric line and the base.

A conductive connecting member I05 is me-' chanically supported by baseI04 but is electrically insulated therefrom by means of a glass bead orseal I06 which rigidly supports member I05. At one end of member I05 weprovidea flexible connection such as a coil spring I01 attached to theinner conductor 83. I

The base I04 is also provided with a threaded flange I08 which supportsa metallic thimble I09 provided with a shoulder I I0which'longitudinally positions the cathode positioning or centeringmeans to be described immediately. This oath ode centering means isflexibly connected to member I05 by means of a flexible connection suchas a coil spring III and comprises an annular centering ring II2 havingnear the periphery thereof a restricted region II3 to'obtain asubstantial line engagement with shoulder I I0. Ring II2 may bepositioned by a suitable mechanical expedient such as a plurality ofcircumferentially displaced screws II4. Furthermore, the centering ringH2 is maintained in the desired longitudinal or axial position in firmengagement against shoulder IIO by means of a compression spring II5which is supported atits right-hand extremity by means of a threadedcollar II6 which engages threads on the inner surface of thimbl I09. Asa means for insulating the shell of thimble I09 and the centering meansfrom the center conductor to the cathode, we employ a cylindricalinsulator III placed between retaining ring I12 and a cylindricalcathode base-H8 adapted to receive a rigid cath'ode stem II9. Oneterminal I20 of the point cathode 65 is connected to stem I I9 through aface plate I2I which is attached to stem II 9, the latter constitutingan electrical path for the inner conductor 83 through coil spring III,connecting member I05 and coil spring I01. The other terminal of thecathode I22 is connected to a metallic pin I23 supported by andelectrically insulated from the face plate I2I by means of a glass beadI24.

As a means for facilitating ease of replacement of the cathode element65 and to permit mechanical movement of the stem II 9 and cathode 65without. disturbing the electrical connections to the cathode, we employa flexible conductorsuch as a ribbon conductor I25 which extends into anopening "I26 in the body of thimble I09. A screw I21 or any suitablemechanical expedient may be employed for retaining and removing theribbon'conductor I25 in opening I26; The electrical, path from terminalI22 of cathode 65 comprises pin I23, conductor I25, thimble I09 andtubular conductor 84 of the concentric line. At the end of thimble I09we provide a cap I28 which is removable or demo-untable and which isadapted to be supported by thimble I09 by a screw-threaded engagement atI29." This cap I 28 is provided with an aperture I30 through which theelectrons emerge upon acceleration due to the field produced by anodecup I3.

Analternative flexible connection for the oathode supporting structureis shown in Fig. 5 and corresponding elements have been assigned likereference numerals. Instead of employing coiled springs as the flexibleconnection and extending one of the springs through member I I8, we mayemploy folded strips I3I and I32 of resilient metal, respectively,connected to members II8, I05 and I05, 83.

The actuating mechanism for releasing the pressure on cover 56 of thevalve 55 closing the specimen insertion chamber is shown in Fig. 6, Rod51 which exerts pressure against cap 56 may be removed therefrom bymovement of h'andie I I in a clockwise'direction, causing the clockwiserotation of a pivoted actuating arm I33 which is pivoted at some pointto the right of the figure and which is connected to rod 51 through acrank I34. Member 63 which actuates valve 62 is also connected toactuating arm I33 by means ofa connecting rodI35 which is positioned toactuate valve 62, thereby admitting air to the insertion chamber and themain evacuation chamber so that the cover 56 may be removed.

In Fig. '7 there is diagrammatically illustrated, partially inperspective and partially in cross section,fthe unitary electron lensassembly showing the plurality of spaced electrostatic lens systems 30,3I and 32, and corresponding elements have been assigned like referencenumerals.

Referring to the electron lens 32, it will be noted that theintermediate electrode 49 thereof, which corresponds to the intermediateelectrodes 31 of electron lense 30 and 3|, is supported by an insulatorI36 which holds the intermediate electrode 49 firmly in position andalinement. Provision is made for the impression on the intermediateelectrodes of each lens system of a potential corresponding to theoathode potential by means of the conductor shown in Fig. 13.This-conductor extends longitudinally of the electron gun and is placedwithin the cyl- These apertures are also finely machined or ground tohave convex peripheries to minimize high electric field gradients.

The conductor is a sectionalized conductor to facilitate assembly anddisassembly of the elec-i tron lens unit, and comprises a plurality ofconductors I42, I43 and I44 which are welded or otherwise attached tothe intermediate electrodes and are provided with recesses to receivetelescoping intermediate members I45 and I45.

There is also provided within the electron lens assembly electrostaticshielding means spaced lon= gitudinally between the sectionalizedconductor and the electron path. This shielding means may comprise aplurality of planar metallic shielding members I4! and I48 which closelyengage the inner surface of cylinder 29 and abut facing outer electrodesof the electron lenses 30-32, in-

clusive.

As means for facilitating assembly of the'entire electron lens unit andfor positioning the actuating rods 93-95 in a position which ismechanically expedient and which will not deleteriously affect theoperation of the electron lens system, we provide longitudinal grooveslying incylinder. 29 and adapted to receive and permit longitudinalmovement of rods 9395. Only one of these grooves, that is groove I49which receives rod 93, is shown in Fig. '7. Each of rods 9395 isterminated in a. foot I50 which engages heads 99 of the screw-threaddevices 96-98 shown in Fig. 3.

As a means for looking or keying the electron lens assembly in thedesired or fixed angular position within cylinder 29, there may beemployed suitable means such as a tongue II constituting an integralpart of or attached tothe cylinder,

and which may be adapted'to engage one of the motion is employed forfocusing purposes, the

latter being necessary in view of the fact that the focal length of theelectrostatic lens system provided by the microscope is fixed by thegeometry of the electrodes or lenses thereof.

The manipulator shown in Fig. 8 is provided with three actuating leversI52, I53 and I54, only two of which are shown in Fig. 8. The details ofconstruction and operation of the manipulator will be explained inconnection with Figs. 14-29, inclusive. As to the general featuresofoperation, it may be said here that the specimen carrier 45 is supportedat one end by an annular ring I55 constituting a part of themanipulators. and this ring by virtue of the aforementioned ways movesthe carrier 45 for scanning and focusing purposes. The entiremanipulator is supported by a face plate I55 which is maintained in thedesired axial position by engagement with the inner surface ofcylinder29.

Fig, 9 is a detailed cross-sectional view of the specimen carrier 45which may be of the cartridge type comprising a retaining shell I5Ihaving a longitudinal opening I58 through which. the electron beampasses, and having aresilient holding or positioning member such as aplunger;

I59 alsoapertured. The plunger I 59 may be biased by means of acompression spring I50. The carrier I45 'is' adapted to receive a locketshown in Fig. 12 which holds a specimen, and the distance between theinner vertical surface of shell I51 and the face of plunger I59 is suchthat aslight pressure must be exerted upon the locket toobtain aninsertion and in order firmly to hold the locket during adjustment ofthe mani ulator.

In Fig. 10 there is illustrated the structure for supporting the beamlimiting disk 50. This structure comprises a face plate IBI which issupported from face plate I55 by means of a plurality ofspaced'supporting rod I62 and I63. Other rods, not shown, may also beemployed to afford a firm support. Molybdenum disk 55 is held inposition by a shroud ring I54, the latterof which is positioned by aplurality of thumbscrews I55 which may be adjusted to obtain the desiredalinement of the aperture in disk 50 with respect to the electron gunand the apertures of tlieelectrodes constituting the electron lenssystem. Thumbscrews I55 are supported by an annular memberj I65 having acentral aperture within which shroud ring I 64 and disk 53 arepositioned and is provided with a clamp I61 to control the axialdisplacement of these members. Face plate I5I and annular member I65 areprovided with apertures I69 and I58 which, however, for the purpose ofconstruction and arrangement are not in alinement with the correspondingapertures I3'I-I4I in the disks or electrodes of the electron lensassembly or aperture I'IIl of face plate I55 shown in Fig. 8. Theselatter mentioned apertures are provided for the purpose of receiving thesectionalized horizontal conductor shown in Fig. 13 and which will bedescribed in detail hereinafter.

A specimen. holder is shown in Fig. 11 and is arranged to be insertedthrough the specimen insertion chamber 54 shown in Fig. 3 and into slot'5I and includes a locket III comprising a recessed ring I'IZ adapted toreceive a fine removable' mesh upon which the specimen is placed andalockingand retaining ring I'I3. Rings I12 and H3 are shownincross-sectional detail in Fig. 12.

' In Fig. 11 there isalso shown a suitable specimen holder whichcomprises a stem I'I4 spring biased to the position illustrated withrespect to a cylinder I'I5. Stem I'I4 telescopes cylinder I15 and thelatter is attached to a curved holding plate H6 whereby spring I'I'I',in the absence of pressure 'on knob I18; draws the stem I'I5 upward. Therecesseddisk H2 is flexibly connected" to the stem I'I4 by suitablemeans such as a fine-link. chain I19. The lower end of stem I'lltg'is'provided with a curvature to engage the p'eripheryof disk In upondepression of member I118 thereby firmly. holding the locket inengagement for the purpose of extending and positioning theJocket-incarrier 45 shown in Fig. 9.

In Fig. 13 there is shown a portion of a longitudinal sectionalizedconductor for impressing cathode. potential on the intermediateelectrodes of the various electron lenses 3II--32 and correspondingelements have been assignedv reference numeralsem-ployed in Fig. 7. Dueto the fact that the apertures I68 and-I69 of the beam limitingstructure are not in alinement with apertures I3] I4 I ofthe, lensassembly and aperture IIll oflthe manipulator face plate" I55, weprovide an elbow I89; which; telescopes section I42 of the conductorandwhich is provided with a highly pclishedand finely ground cup IBIadapted tov receive a resiliently mounted pin I92 constituting a part ofsection I83 and which, in turn, telescopes a second elbow section I84which is in electrical contact with and received by thimble I09 of thecathode supporting structure shown in Fig. 4.

Referring now to the details of the specimen manipulator shown in Fig.8, reference is here had to the exploded view of the manipulator shownin Fig. 14. The only elements there illustrated are the face plate andsupporting ways. The manipulator per se is disclosed and claimed inUnited States Letters Patent No. 2,380,209, granted July 10, 1945, toCharles H. Bachman and Jacob Tschopp.

There is provided on face plate I56 a pair of vertical ways I 95 and I86having therein races I91, preferably of V-shaped configuration tosupport a plurality of friction reducing bearings such as miniature ballbearings I88. A vertical transversing member I89 is also provided withraces I90 and I9! to engage the ball bearings I 98. Attached to thetransversing member I89 there is provided a pair of horizontal ways I92and I93 also comprising V-shaped races I94 and I95 adapted to receiveminiature ball bearings I96. A horizontal transversing member I91provided with suitable races I98 and 199 engages the ball bearings I96.A third set of ways, namely, the horizontal or axial ways for focusingpurposes comprising ways 200 and 20I, are supported by the horizontaltransversing member I91 and also comprise V-shaped races 202 and 209likewise provided with miniature ball bearings 204 for reducingfriction. Ring I55 constitutes a part of an L-shaped member 205 providedwith races to engage the ball bearings held by races 202 and 203 and isultimately supported by ways 200 and 29I.

Appreciation of the manner of operation of the manipulator shown indetail may be obtained by referring to Figs. 1.5-20, inclusive. Figs.15-20, inclusive, do not show all of the elements of the 14 comprise aspring 2I2a. Forward motion, that is a motion to the left of lever I54viewed from Fig. 18, raises ways I92 and 193 thereby raising ring I55and the associated carrier 45.

The manner in which the focusing or longitudinal movement of ring I55 isobtained may be appreciated by referring to Figs. 19 and 20. Actuatinglever I53 is connected to member 3205 by means of a crank or connectingrod 2I3 and a cam 2M, the latter of which engages member 205 to move ithorizontally or longitudinally. The spring 2 I21) serves to preventback-lash.

The structure through which the actuating wheels, including illustratedwheels I and 0, for

manipulator 95. The associated groups I5, I6; I'I,

I8 and I9, 20 show only those elements deemed necessary in order toemphasize the various independent movements produceable by the manipbyactuation of lever I52, indicate that upon movement of lever I52forward, that is, away from the drawing, lever I52 actuates crank 206(shown in Fig. 8) which in turn rotates connecting rod 297 to move cam209, thereby exerting a pressure against the structure for traversingmember I91, and thereby moving the horizontal transversing member I99 tothe right. There is also provided means in connection with each of themovements for substantially eliminating or reducing back-lash which, inconnection with the arrangements shown in Figs. 15 and 16, comprises aspring 299 which biases member I99 towards the left observed from thefront view shown in Fig. 15.

Operation of the vertical transverse motion produced by the manipulatormay be obtained by referring to Figs. 17 and 18- which are,respectively, front and side views showing additional structure forobtaining this motion. Lever I54 is connected to rod 2 I 0 which in turnoperates a cam 2II connected to the way I92.

Back-lash prevention means is also provided in connection with member202 to bias this member controlling the manipulator are joined to andoperatively associated with the screwthread devices may be more fullyappreciated by referring to Fig. 21. Certain of the elements thereillustrated have been described hereinbefore and corre-' spondingelements have been assigned like reference numerals. The end plate ordisk 96 shown at the left-hand part of Fig. 3 is presented in enlargedform in Fig. 21 and shows the manner in which the contact or connectingpins IIJI extend through that disk affording suitable connections for aplurality of flexible cables 2I5, 2I6 and 2II which are provided withadaptors 2I8 and which are connected to the knurled wheels I, 8 and 9.

A viewing aperture mounted at the end of the horizontally disposed coverfor the electron gun, and represented by the reference numeral 6 in,Fig. 22, may be designed to be removable andv comprises an eye-piece2I9, a light or glass lensv 220 supported by a cylinder 22I which isattached to eye-piece 2I9 and which is insertable within a holdingcylinder 222. The light lens 220 is in alinement with the fluorescentscreen provided by disk shown in Fig. 21 and serves to magnify the imageappearing on this screen.

An arrangement is provided whereby in place of using a light magnifyinglens a camera 223 may be supported at the end of the cover cap 5 asillustrated in Fig. 23. In such instances, the camera 223 which may alsoinclude a magnifying lens is provided with an adaptor 224 which isinsertable within holding cylinder 222 and may alsobe provided withhorizontal supporting members; 225 and a further supporting member 226.

Fig. 24 diagrammatically illustrates an electric: circuit diagram whichmay be employed for the. application of anode-cathode voltage to theelec-- tron microscope and for the control of the oathode heatingcurrent. The electron microscope which we provide is capable of beingenergized from any suitable source of alternating current such as thatreadily available at the usual commercial power outlets. For example,the system may be energized from an alternating current circuit 22'Iofcommercial frequency. The main switch 22 shown in Fig. 1 may beconnected in the position illustrated to control the application ofvoltage and current to the microscope as well as to control theapplication of power to the pumps I6 and I! and centrifugal blower I5. Aunidirectional voltage is produced by means of a rectifier circuit whichmay include a single unidirectional conducting device such as anelectric discharge device 228 which is energized from a transformer 229.The voltage controlling means I3 may comprise means, such as a rheostat230, connected in series relation with the primary winding thereof tocontrol the magnitude of the unidirectional voltage applied to themicroscope and may be actuated by knob 20 shown in Fig. 1. One terminalof the secondary winding of trans-- 15.. former 229 is connectedtoground. By virtue of the polarization of the discharge device 228,conductor 2-3I is maintained at a negative potential with respect toground. Current limiting resistances 232 and 233 may be connected in thepositions illustrated not only to limit the current transmitted bydischarge device 228 but also to limit the current supplied to theanode-cathode circuit. of the electron microscope. In order to effectthe application of a substantially constant unidirectional voltage tothe microscope, there may be provided suitable filtering means such as acapacitance 234' which is connected between ground and the commonjuncture of resistances 232 and 233.

Transformer 235 is employed as a source of cathode heating current orfilament current for the discharge device 228. Variation of the cathodeheating current supplied to cathode 65 of the electron gun may beobtained by any suitable arrangement such as a transformer 236controlled by means such as a rheostat 231, the latter of which may beactuated by switch or dial 21 shown in Fig. 1.

Conductors 238 constitute means for supplying cathode heating current tothe concentric line comprising inner conductor 83 and the tubular outerconductor 84 shown in Fig. 3. An adaptor (not shown) may be employed forconnecting conductors 23l and 238 to the elements of the electron gunthrough a shielded cable 239 comprising a grounded metallic shield 240,conductors 238 which extend therethrough centrally, and a tubularconductor 24L the latter of which is connected to the outer tubularconductor 84 of the concentric line.

In operation, the specimen or object to be investigated is placed upon afine wire mesh which is inserted within the recess of ring I12constituting one part of locket I'll shown in Fig. 12, and the lookingor retaining ring I13 is employed to maintain the mesh firmly againstthe inner shoulder of ring H2. The holding plate H6 is grasped and theknob H8 is depressed, thereby extending the rigid stem I'M so that thecurved lower portion thereof firmly engages the outer periphery of ringH2, thereby positively holding it. Cover 550i Valve 55 is removed andthe specimen holder is extended through chamber 54 and slot and thelocket is inserted in specimen carrier 45, particularly between theinner vertical surface of shell l5! and the face 1'6 ode'voltage may beapplied to the electron gun by operation of knob or dial 20 and thefilament current may be controlled by knob 2!, both of which areillustrated in Figs. 1 and 24. Of course, rheostat l3 permits adjustmentof the voltage applied to the anode and cathode of the electron gun, andcorrespondingly controls the magnitude of the voltage impressed acrossthe outer electrodes and the intermediate electrode of each-of theelectrostatic lenses 30-32.

The electron image formed at the specimen plane is-magnified by theelectrostatic objective the eye-piece 6 which, in turn, position heads99 of plunger I59, the latter serving to hold the locket firmly. Theknob H8 is then released which permits retraction of the stem I14 upwardinto cylinder I15, which action renders the locket Ill free to bepositioned by the carrier 45 and manipulator 4B. The holder is thenpermitted to rest against the side or vertical wall of insertion chamber54, the length of the holder being such that it does not extendsufliciently upward to prohibit the closing or placement of the cover56.

The valve is then sealed by placing the transverse rod 57 in the grooveprovided in cover 56, and handle ll of the valve i moved in a clockwisedirection, viewed from Fig. 6, to apply sealing pressure to the cover.

Evacuation of the chamber enclosing the electron gun, the lens systemand other associated elements is then initiated by the operation ofhandle or knob 23 shown in Figs. 1 and 24. After the pressure within thechamber has been reduced to a suitable value, power may be applied tothe electronmicroscope; that is, anode-cathof the screw-thread devices96-98, inclusive, which are operatively connected to levers 152- I54 ofmanipulator 46 through rods 53-95.

The specimen may be removed in the following manner: Handle H shown inFig, 6 is raised effectihg release of the pressure on cover 56 so thatthis cdver may be removed. However, prior to the removal of the cover,as stated above, heated air'is admitted to the insertion chamber 54 andto the main evacuated chamber through passage 65 by means of valve 62which is actuated by arm I33,- connecting rod I35 and member 63. As soonas the differential between atmospheric pressure and the pressure withinthe chamber is suifi'ciently reduced, cover 56 is then readilyremovable.

It has been found that the limiting angle of the electron bundle, orbundles, leaving the objective lenses 38-32 can be established mainly bythe electron gun aperture rather than by the apertures of the objectivelenses. This type of limitation as to the divergence of the electronbeam affords distinct advantages from a mechanical: standpoint becausethe last aperture in the electron. gun preceding the specimen determinesthat angle, and that last aperture may be more favorably located thanthe objective lens stop, thereby avoiding the necessity for providingextremely small diameter holes in the electrodes constituting theelements of the various electrostatic lenses. Although the lensaberrations as well as the. particular thickness and material ofthespecimen under examination play an important part in determininwhether the gun angle may serve as the effective stop of the system, orwhether-that limit is best determined by the first objective lens, wehave found that by the optimumplacing of the electron gun aperture at anappreciable longitudinal or axial distance from theobject, ,a highlysatisfactory structure is provided for producing this limiting efiect.In other words, we position the electron gun so that itestablishesa-relatively small gun angle,

Although we do not show in the modifications of our 'inventionillustrated a separate condenser lens; for focusing electrons on thespecimen, the use of such a condenser lens is optional in the electronmicroscope which we provide. The electron gun, since it is a means forperforming such focusing operation, includes in it the focusingproperties of a condenser lens, and we place the electron gun' at anoptimum position to utilize to ajgreat extent the condensing effect ofthe :electron chamber, an electron lens system within said chamber,means for supporting said electron gun and chamber in the horizontalposition, aviewing aperture at one end of said chamber, means withinsaid chamber for positioning :a specimen to be investigated, actuatingmeans within the vicinity of said aperture for controlling the lastmentioned means, and a substantially vertical specimen insertion chamberin communication with .the first mentioned chamber.

5. In an electron microscope, the combination including a horizontallydisposed electron gun chamber, an electron lens system within saidchamber, means for supporting said electron gun and chamber in ahorizontal position, a viewing aperture at one end of said .chamber,means within said chamber for positioning a specimen to be investigated,a specimen insertion chamber in communication with the first mentionedchamher, and externally accessible means supported in the vicinity ofsaid aperture for adjusting said specimen positioning means in threemutually perpendicular directions whereby the positioning of saidspecimen in accordance with the image produced at said aperture isfacilitated.

6. In an electron microscope, the combination including a horizontallydisposed electron gun chamber, a cabinet for supporting said chamber andcontaining therein a power supply for the electron gun, an evacuationmeans for reducing the pressure within said chamber, a light opticalviewing aperture at one end of said chamber, a horizontal table partattached to said cabinet below said chamber, means within said chamberfor positioning a specimen to be investigated, actuating means Withinthe vicinity of said apertureforcontrolling the last-mentioned means, a

specimen insertion chamber in communication with said first mentionedchamber, and means supported by said cabinet above said table part forcontrolling the power supply to the electron gun. f

CHARLES H. BACHMAN. SIMONRAMO.

REFERENCES CITED The following references are of record in the file of;this patent:

UNITED STATES PATENTS Number Name Date 2,360,872 Hillier Oct. 24, 19442,052,914v Rudenberg Oct. 27, 1936 2,132,076 Kotraschek et a1. Oct. 4,1938 2,215,979 Schuchmann Sept. 24, 1940 1,419,241- Edwards June 13,1922 OTHER REFERENCES

